Field of Invention
The present invention relates to substrate films for an optical sheet, optical sheets, and backlight units capable of markedly enhancing utilization efficiency of rays of light and improving luminance.
Description of the Related Art
Liquid crystal display (LCD) devices have been frequently used as a flat panel display through utilizing features of thinness, lightness in weight and low level of electric power consumption, and applications thereof have been increasing year by year. The liquid crystal display devices in widespread use have involved a backlight system where light emission is executed by irradiating onto a liquid crystal layer from the back face. In such a type of display devices, a backlight unit which is an edge light type, an immediate beneath type or the like is provided to an under face side of the liquid crystal layer. Such a backlight unit 50 of an edge light type is equipped with a rod-shaped lamp 51 for use as a light source, an optical waveguide plate 52 having a square plate shape disposed so that the edge thereof abuts along the lamp 51, a light diffusion sheet 53 disposed to the front face side of the optical waveguide plate 52, and a prism sheet 54 disposed to the front face side of the light diffusion sheet 53, as fundamentally shown in FIG. 9A.
Referring to functions of this backlight unit 50, rays of incident light from the lamp 51 to the optical waveguide plate 52 are first reflected on reflection dots or a reflection sheet (not shown in the Figure) of the back face of the waveguide plate 52, and exit from the front face of the waveguide plate 52. The rays of light exited from the waveguide plate 52 enter into the light diffusion sheet 53, then are diffused by the light diffusion sheet 53 and exit from the front face of the light diffusion sheet 53. Thereafter, the rays of light exited from the light diffusion sheet 53 enter into the prism sheet 54, and exit as rays of light having a distribution representing a peak in an orientation along a substantially normal line via a prism part 54a formed on the front face of the prism sheet 54.
Accordingly, the rays of light exited from the lamp 51 are diffused by the light diffusion sheet 53, and refracted by the prism sheet 54 so that they represent a peak in a direction along the substantially normal line, and illuminate the entire face of the liquid crystal layer on the front face side (not shown in the Figure). Meanwhile, although not shown in the Figure, a light diffusion sheet may be further disposed to the front face side of the prism sheet 54 for the purpose of: relaxation of light condensing properties of the prism sheet 54 as described above; protection of the prism part 54a; or prevention of the sticking between the prism sheet 54 and the liquid crystal panel such as a polarizing plate.
The light diffusion sheet 53 to be disposed in the backlight unit 50 generally has a transparent substrate layer 56 made of a synthetic resin, and a light diffusion layer 57 overlaid on the front face of the substrate layer 56 as shown in FIG. 9B (for example, see, JP-A Nos. H07-5305 and 2000-89007). In general, this light diffusion layer 57 includes resin beads 59 in a transparent resin binder 58, and the beads 59 exert the light diffusion function.
In recent years, characteristics required for LCD vary depending on their application, but may include brightness (high luminance), visibility (widening of viewing angle), energy saving, thin and light modeling capability and the like. Particularly, high luminance has been inevitably desired. However, in order to secure the luminance while keeping the concept of LCD, i.e., thin and light modeling, merely improving the optical waveguide plate 52 and the lamp 51 in the backlight unit 50 such as a cold-cathode tube is not satisfactory under current circumstances.
In LCD, a polarizing plate serves as a requisite essential member. Generally used polarizing plates are those which exhibit absorption dichroism, i.e., absorption of one directional component of a light accompanied by transmission of remaining polarization components. This type of the polarizing plate absorbs 50% of the light, in principle, for achieving polarization, therefore, it falls under one great reason for deterioration of the utilization efficiency of the light in LCD.
Recently, in order to enhance the utilization efficiency of the light, a reflection polarizing plate is disposed at an upper position in the backlight unit 50. This reflection polarizing plate allows a transmission axis component of the polarizing plate to transmit directly, while other polarization components are allowed to return to the lower side, whereby recycling the rays of light. However, also according to the backlight unit 50 in which such a reflection polarizing plate is laminated, approximately 75% of the utilization efficiency of the rays of light can be achieved in fact due to loss of the recycled light through thermal absorption, reflection and the like.